The invention relates to a semiconductor photodetector for optical communications, optical information processing, optical measurement and the like, and more particularly to an ultraspeed, low-voltage drive avalanche multiplication type semiconductor photodetector.
PIN type semiconductor photodetectors comprising a light absorbing layer of In0.53Ga0.47As (hereinafter referred to as xe2x80x9cInGaAs layerxe2x80x9d) provided in a lattice matched form onto an InP substrate (Yonedu, xe2x80x9cHIKARI TUSHIN SOSHI KOGAKU (OPTICAL COMMUNICATIONS DEVICE ENGINEERING),xe2x80x9d Kougaku Tosho Co., Ltd., 371 (1983)) and avalanche multiplication type semiconductor photodetectors, Electronics Letters, Vol. 20, 653-654 (1984) are known as conventional semiconductor photodetectors for optical communications in a 1 to 1.6 xcexcm wavelength range. In particular, by virtue of internal gain effect and high-speed response derived from avalanche multiplication, the latter has been put to practical use for long distance communications.
A typical InGaAs-APD (the avalanche multiplication type semiconductor photodetector being hereinafter often referred to as xe2x80x9cAPDxe2x80x9d) will be explained. The InGaAs-APD is operated as follows. Among photocarriers generated in the InGaAs light absorbing layer, holes are injected by the electric field into the InP avalanche multiplication layer. Since a high electric field is applied to the avalanche layer, the holes are accelerated and give rise to impact ionization. Noise characteristics and high-speed characteristics important for the characteristics of APD are governed by random ionization process of carriers in the course of multiplication. More specifically, a higher ratio of the ionization coefficient of electrons and the ionization coefficient of holes in the multiplication layer leads to expectation of lower noise characteristics. Further, the time taken for providing a predetermined multiplication factor can be shortened with increasing the ionization coefficient ratio, and in this case, high-speed characteristics can also be attained. Electron ionization coefficient a may be larger than hole ionization coefficientxcex2, or vice versa. A larger ratio xcex1/xcex2 (or xcex2/xcex1) offers better results.
The ionization coefficient ratio, however, is inherent in materials, and, in the case of InP as the multiplication layer in the InGaAs-APD, is about 2 at the highest in terms of xcex2/xcex1. On the other hand, in the case of Si-APD used at 0.8 xcexcm wavelength, the ionization coefficient (xcex1/xcex2) of the Si multiplication layer is as large as about 20 to 50, and hence can offer satisfactory low noise characteristics. For this reason, development of APDs having a high ionization coefficient ratio even in a long wavelength range, i.e., a 1 to 1.6 xcexcm wavelength range, has been desired in the art.
On the other hand, in recent years, research reports have been made on superlattice APDs and staircase APDs having a superlattice structure and a multilayered structure with a graded composition in the multiplication layer. These APDs have been prepared with a view to accelerating the impact ionization of electrons using conduction band discontinuous energy and increasing the ionization coefficient ratio. The gain bandwidth product for superlattice APDs using an InAlAs/InAlGaAs superlattice layer as the multiplication layer has been reported to be 120 GHz which is on a level high enough for practical use of the APDs (IEEE Photonics Technology Letters, Vol. 5, 675-677 (1993)). Further, for staircase APDs using, as the multiplication layer, a multilayered structure with the composition thereof being graded from InAlAs to InAlGaAs, an ionization coefficient ratio exceeding the superlattice APDs has been reported (Appl. Phys. Letters, Vol. 65, 3248-3250 (1994)).
On the other hand, a plurality of proposals have been made for increasing the ionization coefficient ratio on a principle different from the above principle. One of them is a graded gap APD (for example, Int. Symp. GaAs and Related Compounds, Japan, 473-478 (1981)). In this APD, a multiplication layer with the band gap energy being continuously graded is used to travel electrons from the wide gap side to the narrow gap side. The impact ionization is known from experience to occur when the carrier has obtained energy in an amount of at least 1.5 times the band gap. Therefore, electrons being traveled toward the narrow gap side are likely to cause impact ionization. On the other hand, holes are traveled toward the wide gap side, and hence is less likely to cause impact ionization. This leads to expectation of an improvement in ionization coefficient ratio. Here the graded band refers to the presence of internal pseudo-electric field. Therefore, energy in a larger amount than the externally applied electric field is given to the electrons. In this research report, a device having a 0.7 xcexcm-thick multiplication layer of AlxGaIxe2x88x92xAs is compared with a 0.4 xcexcm-thick multiplication layer of AlxGaIxe2x88x92xAs to demonstrate that the latter has a larger ionization coefficient ratio. Theoretically, since the internal electric field effect of the conduction band is increased, the multiplication factor of electrons must be increased. According to experimental results, however, there is no significant change in multiplication factor of electrons, and the multiplication factor of holes is suppressed, resulting in increased ionization coefficient ratio. There are no experimental reports on such APDs.
As described above, an attempt to improve the ionization coefficient ratio has been made on APDs at long wavelengths. As described in the above report, superlattice APDs and staircase APDs capable of coping with a frequency band of 10 Gb/s could have been realized. In these devices, however, the operation voltage is not less than 20 V, and the suppression of ionization coefficient ratio by the application of high electric field occurs. Consequently, the ionization coefficient ratio is about 3 to 6 at the highest.
Under the above circumstances, the invention has been made, and it is an object of the invention to provide an avalanche multiplication type semiconductor photodetector that can simultaneously realize low-voltage operation and high ionization coefficient ratio.
According to the first feature of the invention, a avalanche multiplication type semiconductor photodetector comprises: a semiconductor substrate; and at least a light absorbing layer and a multiplication layer provided on said semiconductor substrate, the light absorbing layer comprising InGaAs, the multiplication layer comprising In(1xe2x88x92xxe2x88x92y)AlxGayAs with the composition thereof being graded so that the energy band is continuously decreased from the light absorbing layer side toward the multiplication layer in its side remote from the light absorbing layer.
In the semiconductor photodetector according to the first feature of the invention, adoption as the multiplication layer of In(1xe2x88x92xxe2x88x92y)AlxGayAs with the composition thereof being graded permits an electric field exceeding 50 kV/cm to be added to electrons, and occurrence of ionization of electrons in the narrow gap is likely to cause impact ionization. On the other hand, bringing an effective electric field applied to holes to not more than about 200 kV/cm prevents impact ionization of holes. This renders the ionization coefficient ratio close to infinity, so that very low noise characteristics can be realized.
In the semiconductor photodetector, the thickness of the multiplication layer is preferably not more than 0.1 xcexcm.
Preferably, the composition of the multiplication layer is graded from InAlAs to InGaAs.
According to a preferred embodiment of the invention, an electric field relaxation layer of In(1xe2x88x92xxe2x88x92y)AlxGayAs is interposed between the light absorbing layer and the multiplication layer.
Preferably, the composition of the electric field relaxation layer is graded from InGaAs on the light absorbing layer side to InAlAs on the multiplication layer side so as to continuously increase the energy band.
In the semiconductor photodetector having the electric field relaxation layer, preferably, the light absorbing layer is of p-conductive type, the electric field relaxation layer is of p-conductive type, and the multiplication layer is n-conductive type or a high-resistance layer (i-conductive type).
Alternatively, all the light absorbing layer, the electric field relaxation layer, and the multiplication layer may be of n-conductive type or a high-resistance layer (i-conductive type)